Today, St. Louis is a reelection Thunderdome for two political scions. Two men enter, one man leaves.

Democratic Reps. Russ Carnahan and Lacy Clay, Jr. will square off for the whole of the city, which they've shared in Congress for almost a decade, in a primary contest foisted upon them by 2012's congressional redistricting-a process that has already pitted Democrat against Democrat in a handful of districts around the country.

The contest is replete with racial dynamics, negative politicking, and two of Missouri's biggest political family names.

Carnahan, the grandson of Rep. Albert Carnahan, is the son of former Gov. Mel Carnahan and former Sen. Jean Carnahan, who filled her late husband's senate seat when he posthumously defeated Republican John Aschcroft after dying in a plane crash weeks before the election. Since 2005, Russ Carnahan has represented a district that includes the southern half of St. Louis and extends into rural areas along the Mississippi.

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Clay is the son of former Rep. Bill Clay, Sr., Missouri's first African American representative, who served in Congress from 1969-2001. Since 2001, William Lacy Clay, Jr. has represented his father's district, which includes North St. Louis and much of the northern metro area.

Now, the two are vying in Tuesday's Democratic primary for Missouri's newly combined First District, encompassing mostly Clay's territory, with Carnahan's South St. Louis neighborhoods and a few southern suburbs thrown in. Gone are the rural and exurban stretches.

Under the new 2012 map, black and white St. Louis will now share a representative in Congress.

Carnahan, who is white, has represented more predominantly white South St. Louis, while Clay has represented predominantly black North St. Louis. Since 2005, they've shared representation of St. Louis along rough demographic lines that have held for decades.

Carnahan is courting Clay's African American base with a minute-long radio ad that samples William DeVaugh's "Be Thankful for What You've Got" (it charted in 1974, with memorable lyrics, "Diamond in the back , sunroof top, diggin' the scene with a gangster lean…") and a deep-voiced narrator who says, "For Russ, fighting for our families has never been about who's black or who's white. He's all about what's wrong and what's right." The ad is running on St. Louis's urban stations, the St. Louis Post-Dispatch reported. Carnahan has also blanketed the city with mailers that accuse Clay of ties to the Rent-to-Own industry.

Clay, for his part, has advertised his "backbone" against the "hatin' Tea Party" over turntable scratches, a hip-hop/Afrobeat line, and a chorus of "La-cy Claaay!" in a radio ad featuring the voices of his father and former Olympian and East St. Louisan Jackie Joyner Kersee. Clay's campaign has sent trucks through St. Louis neighborhoods blaring a two-and-a-half-minute version of the ad, according to a Clay consultant. ABC verified that over the weekend, a white sedan with Clay decals rolled through St. Louis's Central West End neighborhood, blasting music with its windows down.

St. Louis Mayor Francis Slay and St. Louis County Executive Thomas Dooley, meanwhile, appear in this Clay TV ad citing Carnahan's vote against a progressive budget substitute to the Paul Ryan plan, claiming Carnahan voted"against Medicare" and "caved to the tea party."

The two congressmen are reliable Democratic votes, although Clay is more liberal. In National Journal's latest vote rankings, Carnahan clocked a liberal rating of 70.5, while Clay came in at 93.3.

Whoever wins will almost certainly represent the heavily Democratic city in next year's Congress.